It was 15 years ago that Ted Turner needed something interesting to say in a speech — and decided, in a rush, to give away $1 billion.

“I was on my way to New York to make the speech,” Turner recalled to me. “I just thought, what am I going to say?”

So, in front of a stunned dinner audience, he announced a $1 billion gift to U.N. causes such as fighting global poverty.

In nominal terms, before adjusting for inflation, that semiaccidental donation was, at the time, believed to be the biggest single gift ever made, and it has helped transform philanthropy.

Tycoons used to compete for their place on the Forbes and Fortune lists of wealthiest people. If they did give back, it was often late in life and involved museums or the arts. They spent far more philanthropic dollars on oil paintings of women than on improving the lives of real women.

Turner’s gift helped change that culture, reviving the tradition of great philanthropists like Rockefeller and Carnegie. Turner publicly began needling other billionaires — including Bill Gates and Warren Buffett — to be more generous. That was a breach of etiquette, but it worked.

“It’s a starting point for me of this modern era of high-profile big public giving,” reflected Matthew Bishop, co-author of “Philanthrocapitalism,” a terrific book about how the business world is reshaping philanthropy. “He called on others to step up, which did have a crystallizing effect on others. It allowed journalists and others who were talking to Bill Gates to say: ‘Why aren’t you giving more?’” Then they tormented Buffett with the same question.

Ultimately, Gates and Buffett made huge contributions that are transforming the struggle against global disease and poverty. My hunch is that Gates will be remembered less for his work on personal computers than for his accomplishments against malaria, AIDS and poverty itself.

Gates and Buffett are both now recruiters for the Giving Pledge, which commits zillionaires to give away at least half their wealth. The giving pledge adds to the expectation that those who have won the global jackpot should give something back.

Turner channeled his money through the United Nations Foundation, where it was leveraged to get other contributions so as to bring $2 billion to finance causes from malaria to polio, from climate change to family planning.

The gift brought new respect to the United Nations and made it increasingly fashionable for billionaires to worry about global poverty. These tycoons bring not just their checkbooks to the table but also a business sensibility that introduces greater rigor and evaluation to the world of bleeding hearts.

All this has helped shine a greater spotlight on neglected issues — which, in turn, has led to extraordinary results. A study this month reported that infant mortality around the world dropped by more than half from 1990 to 2010. That’s millions of lives saved each year.

Of course, not everybody has gotten the memo. Take Donald Trump, who has contributed his name to a foundation but little more. An investigation by The Smoking Gun website described him as possibly “the least charitable billionaire in the United States,” for he apparently gave the foundation just $3.7 million — over 20 years. Trump, who has said he is worth $7 billion, is not even the largest contributor to his own foundation.

(A spokesman for Trump suggested that it would be “totally incorrect” to characterize him as uncharitable, saying that he has also donated land in upstate New York for public parks and “millions of dollars” to other causes.)

Turner isn’t shy about encouraging others to jump on board. When I asked if he had any advice for my readers, he grew particularly animated: “You don’t have to have any money to make a difference; you can pick up trash walking down the street, and I do that all the time,” he said. “You can volunteer your time. You can be a big brother or a big sister.”

Look, it makes me a little squeamish to extol a billionaire, for our society already has too much worship of the wealthy — and, in any case, the working poor in America are often more generous in percentage terms (and in volunteering) than those far better off.

That said, it warms my heart that a mogul donated $1 billion to enliven a speech, didn’t even put his name on the foundation and then let the money quietly save lives around the world.